1. Field of the Invention
The present invention relates to an audio signal processing apparatus, and more particularly to processing when muting.
2. Description of the Related Art
A class D amplifier for amplifying audio signals uses a pulse width modulation (PWM) method for input digital PCM signals. In a BTL (Bridged Transformer-Less, Bridged Tied-Load, or Balanced Transformer-Less) connected amplifier, a positive phase PWM signal is output to one channel and a negative phase PWM signal, not GND, is output to another channel to drive a speaker directly or via a demodulation filter.
When performing a mute operation in this class D amplifier, the positive phase PWM signal and the negative phase PWM signal approach the PWM width when muting. However, when re-sampling to convert a quantization bit rate to a lower bit rate and performing noise shaping to shift the distribution of quantization noise to an inaudible range so as to simplify the PWM conversion operation and speed up processing, an amplitude of the PWM output remains when muting and audible noise is generated. Thus, a technique has been proposed for removing the audible noise caused by the noise shaping operation by switching both the positive phase PWM signal and the negative phase PWM signal to 50% duty signals when muting.
FIG. 6 shows a positive phase PWM signal output waveform 100 and a negative phase PWM signal output waveform 200. The PWM output waveforms shown are after demodulation. Furthermore, FIG. 7 shows the signal waveforms 100 and 200 when muting. In the figures, the abscissa represents the time axis and the ordinate represents the amount of deviation from 50% duty of the respective PWM pulse signals. Since the noise shaping effect differs in the positive phase side and in the negative phase side (noise shaper output is determined from input value of past infinite time due to a feedback construction so that the positive phase side and the negative phase side normally have waveforms having completely no correlation), the amount of deviation of the respective signal also varies.
Therefore, when noise removal is attempted by switching the positive phase side and negative phase side signals to 50% duty at an arbitrary timing after muting starts, the duty varies discretely at the switching timing as shown in FIG. 8 and unintended noise (namely, pop noise), which is not shown, results in being generated. Depending on the applied field this noise cannot be ignored.
In Japanese Patent Laid-Open Publication No. 2004-336765, when a mute signal is received in a processing circuit, which outputs to an amplifier a pulse width modulated second signal and a pulse width modulated first signal, which has the same phase or opposite phase with the second signal, a mute circuit outputs a selection signal for outputting a reduced pulse width first signal or a reduced pulse width second signal.
When the PWM resolution is high, noise itself is small so that switching to 50% duty is not necessary and noise is not generated. However, when the PWM resolution is lowered in consideration of cost reduction and mass production, the above-mentioned problem accompanying the noise shaping process becomes significant and annoys the user when muting.